Because an organic EL display is of self-emissive type, it has high contrast and fast response, making is suitable for moving picture applications such as a television for displaying natural images. Generally, an organic EL element is driven at a constant current using a control element such as a transistor, but since the transistor in this case is used in the saturation region, different currents are generated in each pixel, even with the same gradation voltage (voltage step) supplied to the pixel, because of variations in Vth (threshold voltage) and mobility characteristics of the transistor, and it is not possible to keep the emission brightness uniform, which is a problem. In order to solve this problem, a structure having a circuit for compensating for Vth provided inside a pixel is disclosed in WO 1998048403.
FIG. 7 shows the pixel circuit disclosed WO 1998048403. In FIG. 7, a gate of a p-channel switching transistor P4 having a source connected to a data line is connected to a gate line, and the drain of this transistor P4 is connected to the gate of a p-channel driving transistor P1 via a capacitor Cc. The source of the driving transistor P1 is connected to power supply VDD, while the drain is connected via a p-channel light emission controlling transistor P2 and the organic EL element OLED to a negative power supply. Also, a capacitor Cs is arranged between the gate of the driving transistor P1 and the power supply VDD, with a reset transistor P3 being arranged between the gate and source of the driving transistor P1.
With this structure, in a state where the light emission controlling transistor P2 has been turned off by the light emission control line, power supply potential VDD is supplied to the data line, the switching transistor P4 and the reset transistor P3 are turned on by the gate line and the reset line, and Vth is written to the capacitors Cc and Cs. Next, the reset transistor P3 is turned off to apply a data line gradation signal voltage Vsig to the gate of the driving transistor P1 via the capacitor Cc, and a gate voltage Vg=Cc/(Cc+Cs)×Vsig+Vth is applied to the gate terminal of the driving transistor P1.
In this way, since this Vth is always added to the gradation signal voltage as an offset at the gate terminal of the driving transistor, Vth is automatically corrected. However, since a dynamic range of the gradation signal voltage is reduced to Cc/(Cc+Cs), it is preferable to make Cc sufficiently large compared to Cs in order to avoid this. However, if Cc is made larger the surface area occupied by Cc in the pixel section increases, making the surface area of an opening section disadvantageously larger. As a result, the organic EL element is driven at a high current density, and it is difficult to ensure reliability, such as lifespan.
It is also difficult to correct mobility with the Vth correction circuit of the related art disclosed in patent document 1, and it is difficult to ensure high brightness uniformity in a wide gradation range when there are variations in mobility between pixels. Also, an organic EL element generally has reduced emission brightness accompanying light emission, but with the pixel circuit of the related art it is also not possible to correct the lowering of light emission brightness.